Blade structure for an axial flow elastic fluid utilizing machine

ABSTRACT

THE INVENTION PROVIDES AN IMPROVED BLADED ROTOR STRUCTURE FOR AN AXIAL FLOW TURBINE, OR THE OPERABLE AT CONSTANT OR VARIABLE SPEED WITHIN A SPEED RANGE EXTENDING TO MAXIMUM RATED SPEED, WHEREIN THE VIBRATORY STRESS AT RESONANT SPEEDS IS SUBSTANTIALLY REDUCED BY CONNECTING THE BLADES TO EACH OTHER, AS BY SHROUD OR LASHING STRUCTURE IN GROUPS EQUAL IN NUMBER TO THE RESONANT HARMONIC FREQUENCY OF THE BLADES DIVIDED BY THE RESONANT SPEED.

United States Patent Inventors Ralph L Ortolano Saratoga; William P.Welch, Sunnyvale; Harold W. Sennr, Los Altos, Calif.

Appl. No. 809,242

Filed Mar. 21, 1969 Patented June 28, 1971 Assignee WestinghouseElectric Corporation Pittsburgh, Pa.

BLADE STRUCTURE FOR AN AXIAL FLOW ELASTIC FLUID UTILIZING MACHINE 4Claims, 7 Drawing Figs.

U.S. Cl 416/190, 4l 6/l9l,4l6/195 Int.Cl. F01d 5/10, FOld 5/22 Field ofSearch 416/190,

[56] References Cited UNlTED STATES PATENTS 1,366,667 1/1921 Junggren416/191 1,378,464 5/1921 Junggren 416/190 1,639,247 8/1927 Zoelly et a1.416/190 2,454,115 11/1948 Allen 416/189(X) 2,970,808 2/1961 Coppa416/190 3,048,365 8/1962 Foster et a1. 416/190 3,306,577 2/1967 Sagara416/191 Primary Examiner-Everette A. Powell, .1 r. Attorneys-A. T.Stratton, F. P. Lyle and F. Cristiano, Jr.

ABSTRACT: The invention provides an improved bladed rotor structure foran axial flow turbine, or the like, operable at constant or variablespeed within a speed range extending to maximum rated speed, wherein thevibratory stress at resonant speeds is substantially reduced byconnecting the blades to each other, as by shroud or lashing structurein groups equal in number to the resonant harmonic frequency of teeblades divided by the resonant speed.

PATENTED JUH28 197i SHEET 1 OF 2 FIGS.

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INVENTORS y Rolph J. OrtolonqWilliom Ff Welch and Harold W. Semor BLADESTRUCTURE FOR AN AXIAL FLOW ELASTIC FLUID UTILIZING MACHINE BACKGROUNDOF THE INVENTION Because of thermal differences between a turbine rotorand the shroud rings for the rotor blades, it has been the practice ofturbine manufacturers for many years to divide the shroud rings into aplurality of arcuate ring segments, thereby to provide expansion gapsbetween segments. The number of expansion gaps required is thus afunction of the degree of thermal difference expected and the arcuatelength of each shroud segment. In high temperature blading more gaps arenecessary because the blades are short and rigid, and quite highstresses can be developed by small distortions. However, on long lowtemperature bladin'g, fewer gaps are necessary due to greater bladeflexibility and less thermal distortion. Thus; linear expansion of theshroud ring segments is permitted to occur with minimal imposition ofstress on the shroud ring, blades and/or the rotor.

However, with thermal expansion as the only criterion, vibration at oneor more of the blade frequency harmonics has become a problem and, toovercome this vibration problem, the blades have been tuned by laboriousmethods to minimize vibratory stress. At least one such method involvesfiling or otherwise removing metal from the blades to increase theirvibrational frequency to a value higher than encountered within thespeed range of the rotor.

SUMMARY OF THE INVENTION In accordance with the teachings of thisinvention, the vibratory stresses on rotor blades for turbines or thelike may be substantially minimized in the tangential inphase mode byconnecting the blades to each other in groups by lashing structure orshrouding structure divided into arcuate segments of substantially equalarcuate extent and equal in number to the lowest harmonic frequency ofthe blades that it is desirable to suppress. Stated another way, theblades are connected into groups by shroud ring structure or lashingstructure equal in length to the length of one cycle of tangentialvibration of the harmonic frequency that is to be suppressed.

Thus, in general, rotor blades operating at the fourth harmonicfrequency would be provided with four 90 shroud ring segments or four 90lashing structures, while rotor blades operating at the sixth harmonicfrequency would be provided with six 60 shroud ring segments or lashingstructures, etc.

Two of the most common causes of rotating blades vibration are:

l. Excitation induced as they sweep past the Wakes of the stationarynozzle vanes.

2. Excitation induced as they sweep past a local nonuniformity in anadjacent stationary component such as nonuniform nozzle vane spacing, orchange in clearances due to thermal growth or manufacturing distortion.

The first type of excitation of a high frequency excitation andtherefore resonance questions are primarily considered on the small orinitial blades of the turbine which typically have a high vibratingfrequency.

The second type of excitation is most severe when the blades have avibrating frequency that is a low multiple of the rotor speed in cyclesper second. This is the type of blade vibration with which the inventionis concerned.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary view ofaturbine rotor having blades provided with shroud segments in accordancewith the inven tion;

FIG. 2 is a radial sectional view, on a larger scale, taken along linell-Il of FIG. 1;

FIG. 3 is a diagrammatic view of an unshrouded bladed rotor structure,showing the blades in fourth harmonic vibration;

FIG. 4 is a diagram showing the fourth harmonic frequency occurrence inthe rotor of FIG. 3;

FIG. 5 is a chart showing relative amplitude of vibration of the bladesplotted against a ratio of harmonic frequency of rotor running speed tonumber of shroud segments in a blade row;

FIG. 6 is a view similar to FIG. 1, but showing a lashed rotor bladestructure; and

FIG. 7 is a radial sectional view, on a larger scale, taken along lineVII-VII of FIG. 6.

Referring to the drawings in detail, in FIG. 1 there is shown a portionof a turbine rotor 10 comprising a rotor spindle 11 having an array ofradially extending blades 12 supported therein and connected to eachother in arcuate groups by arcuate shroud members or segments 13.Although the entire rotor 10 is not shown, it will be understood thatthe spindle 11 is of circular cross section and the blades 12 arearranged in an annular circumferential array about the rim 14 of thespindle.

The blades 12 may be connected to the rotor rim 14 in any desirablemanner. However, as best illustrated in FIG. 2, the blades are formedwith bulbous l-shaped roots 15 received in a mating peripheral groove 16formed in the spindle rim 14.

The blades are provided with air foil vane portions 17 extendingradially outwardly from the roots l5 and are further provided withtenons 18 extending through uniformly spaced holes 19 and secured to theshroud segments 13 by deformation of the tenons as by riveting.

As thus far described, the structure is substantially conventional, and,in operation as steam or other motive fluid is directed past the turbinerotor blades 12 by the stationary nozzle vanes (not shown) the rotor 10is rotated, as well known in the art. However, during rotation, theblades are susceptible to serious and damaging vibration at frequencieswhich are harmonies of a resonant running speed of the turbine rotor.Two of the most common causes ofblade vibration excitation are:

l. Excitation induced as the blades sweep past the wakes induced by thestationary steam directing nozzles.

2. Excitation induced as the blades pass a local variation in anystationary part associated therewith, such as, for example, nonuniformlyspaced nozzles.

The first type of excitation is a high frequency excitation andtherefore resonant frequencies occurring as high multiples of rotorspeed are primarily considered in short blades, typically formed in theearly stages of expansion in the turbine.

The second type of excitation is a lower frequency excita' tion andtherefore resonant frequencies occurring as low multiples of rotor speedare primarily considered in the longer blades typically found in thelater stages of expansion in the turbine. The invention is concernedwith this type ofvibration.

Both causes of excitation above provide force variations on the rotorblades which are continuously repeated during rotation and when theblade frequency is resonant with the frequency of the force variation,the vibration amplitude of the blades will increase to dangerous levels.When blade resonance occurs, the blade frequency will be a multiple ofthe force variation frequency. This multiple is called the harmonicfrequency and is calculated as follows:

F F1X60 EL H "RPM RPS where:

F the harmonic frequency in cycles per second F the blade fundamentalfrequency in cycles per second R.P.M. turbine rotor speed in revolutionsper minute RPS= turbine rotor speed in revolutions per second.

At any instant, the pattern of deflection of the row of blades 12,without the shroud segments 13, will be a function of some multiple ofturbine rotor speed. For example, if the blades have a fundamentalvibration frequency of 400 c.p.s. and the rotor is rotating at 6000 rpm.or I00 r.p.s., by substitution in the above formula,

F =400/l00 =4; it will be seen that as illustrated in FIG. 4, the bladesare operating in the fourth harmonic F ofthcir fun damental frequencyF,i.e., 400 e.p.s. They will also be vibrating at a frequency equal tofour times the rotational speed of the rotor.

The above phenomenon manifests itself on the unshrouded blades 12 asillustrated in FIG. 3, as four complete cycles of vibration, eachextending across the blades disposed in the arcuate group extendingacross a central angle of 90. At any in stant, one halfof the blades ineach group are deflecting in one tangential direction and the other halfare deflecting in the opposite tangential direction. That is, one halfof the blades are going through half of the fourth harmonic vibrationcycle (-HQF, while the other half of the blades are going through theother halfof the fourth vibration cycle /2F Therefore, if the blades areisolated into four separate arcuate systems or groups, the excitationforces supplied to the blades in each group, above, have a cancellingeffect.

in accordance with the above, the blades 12 are formed into such groupsby connecting them to each other by shroud segments 13 extendingperipherally to the same extent. Thus, the tangential deflecting forceson each of the blades are trans mitted to the other blades through theshroud segments with a cancelling or vibration dampening effect. Todampen the fourth harmonic F four shroud segments of 90 arcuate extentare employed.

If the blades have a fundamental vibration frequency such that at arunning speed of the rotor they are operable in another harmonic, forexample the sixth harmonic, then there will be six vibration waves atany one instant occurring in the blades, each vibration extending acrossthe blades included in 1/6 of 360 or 60 and six 60 shroud segments wouldbe employed to dampen such blade vibrations.

in FIG. 5 there .is shown a chart plotting relative vibrationalamplitude in thousandths of an inch against F /N where F,,= Harmonicfrequency of rotor running speed N Number of shroud ring segments.

In the example given, if: F =4, and N,:4, F,,/N ,=l, and all of theblade vibrations are suppressed, as indicated on the chart.

If the harmonic frequency of rotor running speed is increased, forexample, so that F,,=6, then F IN I .5 and the blades will vibrate at anamplitude of about 2.3 thousandths of an inch, considerably lower thanthe unsuppressed amplitude of l0 thousandths of an inch. Therefore, anyplurality of shroud ring segments, in accordance with the invention,equal to, or lower than, the expected harmonic frequency of rotorrunning speed will result in a ratio of F /N' greater than one, so thatthe blades are never operable in the high amplitude range (below unity).

Although in the first embodiment described above, the blades 12 areconnected into groups by shroud ring segments 13, they may be connectedinto similar groups by other connecting means. FIGS. 6 and 7 showanother embodiment of the invention wherein a rotor structure 50 isprovided with an annular row of unshrouded blades 52 that are connectedto each other in arcuate groups by suitable lashing structure 54 and 55.The lashing structure 54 and 55 connects the blades to each other at twopoints along their radiallength in the embodiment shown. However, thenumber of lashing structures may be modified as desired. For exampleonly one lashing structure may be employed, or alternately, more thantwo, depending on the length and size of the blades, for example. Thearcuate lengthof the lashing structure 54 and/or 55 is selected in thesame manner as the arcuate shroud ring segments to suppress the sameresonant harmonic frequencies of the blades. That is, for the fourthharmonic, four lashing structures of angular extend are employed, etc.

The lashing structure 54 and 55 may be of any type, as well known in theprior art, and in a similar manner, the shroud ring structure 13 may beof any type, as well known in the art.

We claim:

1. A rotor structure for an axial flow elastic fluid utilizing machine,comprising a rotor spindle,

an annular row of radially extending blades carried by saidrotorspindle, and means connecting said blades to each other in arcuategroups of substantially equal central angular extent in degrees,

said blades being susceptible to vibrate in a tangential inphase modehaving a resonant frequency at least twice that of the rated maximumrunning speed of said rotor, and

said connecting means dividing the blade groups into a number equal tosaid resonant frequency divided by said rotor running speed.

2. The structure recited in claim 1, wherein the connecting meanscomprises arcuate shroud ring structure attached to the radiallyoutermost tips of the blades.

3. The structure recited in claim 1, wherein the connecting meanscomprises lashing structure connecting the blades to each otherintermediate their inner and outer ends. 7

4. The structure recited in claim 1, wherein the machine is a steamturbine and is operable at variable speed up to the rated maximumrunning speed.

